1. Technical Field
The invention relates to a method and apparatus for testing transmission lines normally propagating optical signals and is especially, but not exclusively, applicable to a method and apparatus for measuring parameters of optical signals in optical transmission lines of passive optical networks.
2. Background Art
As the cost of optical fiber and associated components decreases, new telecommunications network deployments increasingly use optical fiber from the edge of a core network to a location at or very close to the end user. Such so-called FTTX (Fiber-to-the-X; where X is the home, the office, the building, the premises, the curb, etc.) installations are usually based on a passive optical network (PON) architecture, where a terminal at the core-network edge (Optical Line Terminal—OLT) broadcasts signals downstream along a fiber-optic cable to a N-port splitter, and each of the ports then terminates at an optical network terminal (ONT) located at a respective one of the end users' premises. Typically, downstream signals are at either of two wavelengths, vis. 1490 nm for the downstream transmission of digital data and 1550 nm for the transmission of cable television (CATV) signals, while each end user's optical network terminal (ONT) transmits upstream data signals at a wavelength of approximately 1310 nm. It should be noted that the CATV signals are often transmitted in analog format.
An asynchronous transfer mode (ATM) or similar protocol is often used to encode the downstream and upstream data signals. The OLT includes in the downstream 1490-nm signals synchronization signals which permit each of the ONTs to send its upstream (1310-nm) signals in its own unique time slot so as to avoid interference with signals from other ONTs connected on the PON. For this reason, as well as for reasons of eye safety, there is no 1310-nm transmission from the ONTs when the fiber link is disconnected, thereby preventing reception of the 1490-nm downstream-data signal.
Field maintenance of such FTTX installations requires low-cost and easy-to-use diagnostic test instruments to measure the signals. An example of such diagnostic test instruments is an optical power meter that can independently measure the power at the distinct downstream and upstream signal wavelengths (e.g. 1310 nm, 1490 nm, 1550 nm). During a repair call, the results of such a measurement could indicate the source of possible trouble in the network or in the end-user's connection. It is also known to use optical spectrum analyzers (OSA) to measure optical power at several wavelengths at the same time.
A disadvantage of each of these instruments is that it is a one-port device that only measures the power if the signals at the different wavelengths are propagating in the same direction along the fiber. In the case of the OSA, a further disadvantage is that the instrument is generally much too costly and complicated for routine field applications.